1. Technical Field
This invention relates to a light scattering guiding light source device for constituting a surface light source using an optical element (hereinafter referred to as "light scattering guide") which has a function capable of light-guiding an incident light while being scattered in a volume region, and further relates to a liquid crystal display using the light scattering guiding light source device as a backlight source.
2. Description of Related Art
Conventionally, there have been known various kinds of optical elements and devices of types for emitting a light to a desirous direction using its scattering phenomenon, those of which are used for application such as backlight sources of liquid crystal display devices and the like.
One similar type of those known techniques is a surface light-source in that a light is allowed to be incident from a lateral-side of a transparent material of a extending plate shape, whose one surface side is provided with a reflecting element, and whose other surface has a light scattering property and forms a light emitting surface. Such is described in Japanese Patent Application Laid Opens Sho-62-235905 in 1987, Sho-63-63083 in 1988, Hei-2-13925 in 1990, and Hei-2-245787 in 1990.
In the surface light source using such technique, only a light emitting direction is widened using a scattering reflection and mirror reflection caused on a surface of the transparent material and the reflecting element, and not occurrence of the scattering light within the transparent material in three dimensional manner. Therefore, it is difficult in principle to sufficiently raise a rate of output of the scattering light from the light scattering guiding device.
To obtain the surface light source having a uniform illuminance by lateral side incident light, as is easily understood from examples shown in the respective publications listed above, a reflectivity of the reflecting element and the like are required any of tilt. this provides a complicated larger-sized construction of the light scattering guide device together with a higher cost for manufacturing.
Accordingly, in case where such type of light scattering guiding device is used for the backlight source of liquid crystal display and the like, there must have been sacrificed some from among a brightness, a uniformity of illuminance as a surface light-source, a thin type structure, economy and the others.
Another similar type of the known technique is such that particle shaped substance having a refractive index different from the transparent material is diffused inside the extending plate shaped transparent material.
A light diffusing plate, wherein spherical shaped particles made of silicone resin are diffused into the transparent material such as PMMA, is proposed, for example, in Japanese patent Application Laid Opens Hei-1-172801 in 1989, Hei-3-207743 in 1991, and Hei-3-294348 in 1991. The data of a particle size or particle concentration or the like disclosed in those known documents are used to calculate an effective scattering irradiation parameter E, which will be described later, to obtain a value of 210 to 3000.
Such value E can be employed in case of constituting the light diffusing plate on which, in use, a light is incident from the vertical direction. However, the value E is not suitable for an application of the light scattering guide device of a type capable of taking out a uniform illuminanced light from a light output surface having a spread as is the case of the present invention. Even if the light is incident into the known light scattering member from its lateral-side, a function as the light scattering guide, that means "conductively scattering ranging an appreciable distance", substantially is not provided, this results in illuminating locally only a light output surface extremely in vicinity of a light input surface, it is therefore difficult to realize an uniform surface light source, for example, with a practical size (several cm to several tens cm) as a backlight source of the liquid crystal display.
In addition, an attempt to raise a uniformity of brightness of the light output surface includes Japanese Patent Application Laid Opens Hei-2-221924 in 1990, Hei-2-221925 in 1990, and Hei-2-221926 in 1990. Those are to enlarge the particle size and/or particle concentration depending on increase of distance from the light incident surface, and correspond to enlarge an effective scattering irradiation parameter value E depending on increase of the distance from the light incident surface. However, it is not with ease in technique to practically manufacture such a scattering member, this designates unsuccessful mass productivity and disadvantage in economy. For example, the light scattering guide by batch molding by applying an injection molding technique is hardly obtained.
The Japanese Patent Application Laid Open Hei-4-145485 in 1992 also discloses a light source device which is made of a light guide plate where fine particles are provided into the transparent resin member while a treatment for giving a light diffusing property such as a printing pattern is provided on back surface, where is described "an improvement of a brightness by 2 to 10% is achieved for a light emitting surface of the light guide plate without treatment of the light diffusing for the surface, in comparison with a light guide plate (the conventional light guide plate obtained in the market) whose surface does not contain particles which are processed of the light diffusing treatment". In other words, a technical concept shown and described in the document is that a brightness equal to or more than 90% is generated by contribution by the light diffusing process on back side and a brightness rise of remaining several % is achieved by light diffusing by containing the particles.
The technique disclosed in Japanese Utility Model Registration Patent Application Laid Open Sho-51-89888 in 1976 falls into the same technical field, where data relating to the particle size, particle concentration, refractive index of material, or the like are utterly not disclosed, this is established in accordance with an concept that a brightness and illuminance can be obtained only when a layer having a light diffusing property is provided on back surface of an opaque guide containing particles.
In the prior art as hereinbefore described, when using either of transparent or scattering types of guides, a presently decisive idea is that a brightness of the light output surface is raised by more intensively generating the scattering phenomenon centered a backside surface of the guide, furthermore a uniform degree of brightness as a surface light source is secured by applying a stronger or weaker intensity of the scattering power depending on a distance from the light incident-side through any of means.
However, a difficulty resides in realizing a practical size of the surface light source with a simplified construction having a higher uniformity and level of the brightness. The intensive scattering degree for improving the brightness level results in a difficulty to secure a uniformity of the brightness over a wide area, and in requiring an additional construction with a higher cost for giving a gradient on the scattering power.
A problem difficult to be solved in the prior art, is a directionality of the light which is emitted from the light scattering guiding light source device. For example in considering an application of the liquid crystal display device and the like to a display backlight source, an importance is the illuminance viewed from an observing direction (ordinarily, a limited angle range centered a front direction) of the display device, therefore a strong demand is directed in that a light emitted from the light scattering guiding light source device has a directionality matching with the observing direction. However, as in the prior art described, an approach to secure an illuminance by generating the more intensive scattering centered the back surface of the light guide does not give a clear directionality to the light emitted from the light scattering guiding light source device, it has been impossible to realize the light scattering guiding light source device having a directinality matching to the observing direction of the display device.
The specification of Japanese Utility Model Registration Patent Application Laid Open Sho-51-89888 in 1976 as formerly cited teaches that the backlight source of the liquid crystal display is made of an opaque guide member and a silver colored reflection plate arranged on its back surface side, this however has been argued as a large defected arrangement wherein, as is apparent from a countermeasure in the specification intending to solve the defect by diffusion of the light in a multi-direction through a light scattering layer provided on back-surface of the opaque guide, no description is made for an emitting directionality of the guide itself, no proposition is made for a technical problem of emitting a light flux having an exact directionality in the desirous direction, this disclosure is based on a conception in reverse to the proposition of the light scattering guiding light source device for emitting the light having a sufficient directionality.